EP-A 835 relates to compositions of the formula I and states that X.sup.3 is particularly advantageously K, Rb and/or Cs. In all of the working examples, X.sup.3 is exclusively K. With respect to the preparation of these compositions, EP-A 835 recommends first preparing the mixed oxide EQU X.sub.a.sup.1 X.sub.b.sup.2 O.sub.x
in the absence of the other constituents of the compositions I, mixing this oxide, after its preparation with sources of the other constituents of the compositions I, and drying and calcining the mixture.
Furthermore, EP-A 835 relates to the use of such compositions as catalysts for the gas-phase-catalytic oxidation or ammonoxidation of olefins. However, the multimetal oxide compositions disclosed in EP-A 835 have the disadvantage that, when they are used as catalysts for the gas-phase-catalytic oxidation of isobutene or tert-butanol to methacrolein, the selectivity of methacrolein formation is not entirely satisfactory.
DE-C 3 338 380 discloses compositions of the formula I where X.sup.3 =Cs, but in all cases c&lt;h/20. They are obtained by first mixing a bismuth compound and a tungsten compound in an aqueous medium, drying the aqueous mixture, calcining the resultant composition at from 600.degree. to 900.degree. C. and subsequently powdering the calcined composition so that the particle size is less than 150 .mu.m, mixing the resultant powder with an aqueous solution of the sources of the other constituents of the multimetal oxide composition, evaporating the resultant mixture, shaping and calcining the residue. Furthermore, DE-C 3 338 380 recommends the use of these compositions as catalysts for the gas-phase-catalytic oxidative preparation of (meth) acrolein and (meth) acrylic acid. However, these multimetal oxide compositions disclosed in DE-C 3 338 380 have the disadvantage that, when they are used for the gas-phase-catalytic oxidative preparation of methacrolein from isobutene or tert-butanol, the selectivity of methacrolein formation is not entirely satisfactory.
The earlier application DE-A 42 20 859 (O.Z. 0050/43345) relates to compositions of the formula I, containing three-dimensional regions of the chemical composition EQU X.sub.a.sup.1 X.sub.b.sup.2 O.sub.x
whose maximum diameter is from 1 to 25 .mu.m. This earlier application recommends that X.sup.3 is an alkali metal, thallium and/or samarium. In all the working examples, the alkali metal is potassium. In addition, this earlier application recommends the use of these multimetal oxide compositions as, inter alia, catalysts for the gas-phase-catalytic oxidative preparation of (meth)acrolein. However, the K-containing multimetal oxide compositions of the earlier application DE-A 42 20 859 (O.Z. 0050/43345) have the disadvantage that, when they are used for the gas-phase-catalytic oxidative preparation of methacrolein from isobutene or tert-butanol, the selectivity of the methacrolein formation is not entirely satisfactory.